1. Field of the Invention
The invention relates to a new method of preparing uniformly coated etched glass beads suitable for use as a gas chromatographic column packing. It encompasses the use of a suitable surfactant to promote uniform coating of the liquid phase and minimize thereby tailing of peaks. Within the scope of the invention are included the method of uniformly coating the beads, beads so coated, and the use of the beads in a separation of chromatographically reactive species.
2. Description of the Prior Art
In gas chromatography, separation of gaseous components is accomplished by the presence of a stationary liquid phase coated on a solid support. This solid support may be diatomaceous earth or glass beads, and in the case of open tube capillary columns, may also be the wall of the tube itself. If the solid support is not completely coated and deactivated, potential Lewis acid sites on the support, such as Ca.sup.++ ions, may cause adsorption of lon pair donor molecules such as ketones and amines. When the components are polar, adsorption on an exposed surface occurs readily even in the absence of Lewis acid sites. Adsorption reduces the resolution of the chromatographic separation by causing "tailing" of the peaks. In addition, the literature, e.g., MacDonell, H. L. and Eaton, D. L., Anal. Chem., Vol. 40, p. 1453, (1968), suggests that exposed potential Lewis acid sites on the support may catalyze the decomposition of the gaseous components, particularly at the higher temperatures necessary for the elution of many organic molecules. This also decreases the accuracy of the chromatographic separation.
Thus, it is necessary to achieve a uniform coating on the solid support. Various methods have been employed in attempts to produce such a uniform coating on glass beads, including the filtration method described in Corning Glass Works Product Bulletin, "Data Sheet 102 -- Corning GLC-100 Series Support"; the evaporation of solvent from a slurry of the materials in an open vessel used by commercial chromatographic suppliers; and the removal of solvent using a vacuum rotary evaporator, Filbert, A. M. and Hare, M. L., J. Gas Chromatography, Vol. 6, p. 150 (1968). None of these methods produces uniformly coated beads. As a result, beads coated according to these methods cause poor resolution and selectivity when used as packing in a gas chromatographic column, since adsorption takes place on the uncoated sites. Additionally, the product is not consistent from one lot to the next, so reproducibility of chromatographic results is difficult to obtain.
Under certain conditions (often normal operating conditions), the liquid phase migrates from the solid support, and then interferes with the detection of the gaseous components. This migration is variously referred to as "liquid phase bleeding" or "column bleed". It becomes more pronounced as the temperature is increased, presumably due to thermal degradation of the liquid phase.
The use of sodium silicate beads alleviates to an extent the above problems, since the number of potential Lewis acid sites, e.g., Ca.sup.++ ions, is greatly reduced. However, polar components are adsorbed even where no potential Lewis acid sites are exposed, so tailing still occurs when polar components are separated using a liquid phase coated by conventional means on sodium silicate beads.
It has also been reported (see MacDonell, H. L. Anal. Chem., Vol. 40, No. 1, p. 221, (1968)) that etching the surface of the beads improves their performance as a gas chromatographic column packing. With smooth glass beads the liquid phase is drawn by capillary action to "puddles" at the points of bead contact, leaving only a very thin film of liquid phase on the rest of the bead. This unevenness in the depth of the liquid phase causes the column to have poor separating efficiency. Etching allows a more even distribution of the liquid phase, with a consequent increase in the column's separating efficiency. Etching also allows an increased amount of liquid phase to be coated on the beads, leading to increased column capacity. However, the coating on etched glass beads is not completely uniform, so adsorption occurs, especially with polar compounds, causing tailing of the peaks.
The use of a surfactant in a method of coating a liquid phase on the wall of an open tubular gas chromatographic column has been reported. See Malec, E. J. J. Chromatographic Science, Vol. 9, p. 318 (1971). Significantly, however, Malec teaches only cationic surfactants (quarternary phosphonium halides), whereas cationic surfactants, such as di-isobutyl phenoxy ethoxy ethyl dimethylbenzyl ammonium chloride, were found not to be effective in the present invention. Malec also adds the surfactant, benzyl triphenylphosphonium chloride, to the solution containing the liquid phase, and then applies both the liquid phase and the surfactant at one time. Additionally, the method taught is applicable only to smooth open tubular gas chromatographic columns. There is no suggestion to use the method to produce capillary column packings from etched beads.
Another use of benzyl triphenylphosphonium chloride in a procedure for preparing open tube capillary columns has been described by German, A. L. and Horning, E. C., J. Chromatographic Science, Vol. 11, p. 76 (1973). However, the purpose of the surfactant was chiefly to stabilize the dispersion containing the liquid phase, not to permit uniform coating nor to deactivate active sites on the column wall. Silanization of the glass surface prior to coating with the liquid phase was emphasized as necessary to deactivate the active sites. Both prewetting with solvent (chloroform) and suspension of 6-10 micron particles of silanized silicic aicd in the liquid phase solution were used to effect the desired spreading characteristics and produce thereby a uniform coating. Further, the column treated as described by German and Horning is taught to be sensitive to injections of solvent. As with Malec, the only surfactant taught is cationic, whereas the present invention employs only nonionic or anionic surfactants.